DNA sequencing is a fundamental technology for biology. Several analytical methods have been developed to detect DNA or RNA at the single molecule level using chemical or physical microscopic technologies [15, 16, 21 and 23]. In the past few years, the ion channel has been explored for detecting individual DNA or RNA strands, with nanopore being a candidate for high rate sequencing and analysis of DNA [9, 10, 4, 3 and 7].
In 1996, Kasianowicz et al. first demonstrated that the α-hemolysin channel, an exotoxin secreted by a bacterium, could be used to detect nucleic acids at the single molecule level [8]. The monomeric polypeptide self-assembles in a lipid bilayer membrane to form a heptameric pore, with a 2.6 nm-diameter vestibule and 1.5 nm-diameter limiting aperture (namely, the narrowest point of the pore) [1, 14 and 15]. In an aqueous ionic salt solution such as KCl, the pore formed by the α-hemolysin channel conducts a sufficiently strong and steady ionic current when an appropriate voltage is applied across the membrane. The limiting aperture of the nanopore allows linear single-stranded but not double-stranded nucleic acid molecules (diameter −2.0 nm) to pass through. The polyanionic nucleic acids are driven through the pore by the applied electric field, which blocks or reduces the ionic current that would be otherwise unimpeded. This process of passage generates an electronic signature (FIG. 1) [23 and 5]. A particular nucleic acid molecule, when entering and passing through the nanopore, will generate a characteristic signature that distinguishes it from others. The duration of the blockade is proportional to the length of the nucleic acid, and its signal strength is related to the steric and electronic properties of the nucleotides, namely the identity of the four bases (A, C, G and T).
A specific event diagram is constructed which is the plot of translocation time versus blockade current. This specific event diagram (also referred to as an electronic signature) is used to distinguish the lengths and the compositions of polynucleotides by single-channel recording techniques based on characteristic parameters such as translocation current, translocation duration, and their corresponding dispersions in the diagram [14].
Although the nanopore approach is known as a DNA detection method, this approach for base-to-base sequencing has not yet been achieved.